The electronic energy relaxation of 1-nitronaphthalene was studied in nonpolar, aprotic, and protic solvents in the time window from femtoseconds to microseconds. Excitation at 340 or 360 nm populates the Franck-Condon S(1)(pipi( *)) state, which is proposed to bifurcate into two essentially barrierless nonradiative decay channels with sub-200 fs lifetimes. The first main decay channel connects the S(1) state with a receiver T(n) state that has considerable npi( *) character. The receiver T(n) state undergoes internal conversion to populate the vibrationally excited T(1)(pipi( *)) state in 2-4 ps. It is shown that vibrational cooling dynamics in the T(1) state depends on the solvent used, with average lifetimes in the range from 6 to 12 ps. Furthermore, solvation dynamics competes effectively with vibrational cooling in the triplet manifold in primary alcohols. The relaxed T(1) state undergoes intersystem crossing back to the ground state within a few microseconds in N(2)-saturated solutions in all the solvents studied. The second minor channel involves conformational relaxation of the bright S(1) state (primarily rotation of the NO(2)-group) to populate a dissociative singlet state with significant charge-transfer character and negligible oscillator strength. This dissociative channel is proposed to be responsible for the observed photochemistry in 1-nitronaphthalene. Ground- and excited-state calculations at the density functional level of theory that include bulk and explicit solvent effects lend support to the proposed mechanism where the fluorescent S(1) state decays rapidly and irreversibly to dark excited states. A four-state kinetic model is proposed that satisfactorily explains the origin of the nonradiative electronic relaxation pathways in 1-nitronaphthalene.

Femtosecond transient absorption experiments and density functional calculations are presented for 2-methyl-1-nitronaphthalene, 2-nitronaphthalene, and 1-nitronaphthalene in cyclohexane and acetonitrile solutions. Excitation of 2-methyl-1-nitronaphthalene at 340 nm populates the Franck-Condon singlet state, which bifurcates into two barrierless decay channels with sub-200-fs lifetimes. The primary decay channel connects the Franck-Condon singlet excited state with a receiver triplet state, whereas the second, minor channel involves conformational relaxation to populate an intramolecular charge-transfer state, as previously reported for 1-nitronaphthalene (J. Chem. Phys. 2009, 113, 224518). Conversely, the experimental and computational data for 2-nitronaphthalene shows that almost the entire Franck-Condon singlet excited-state population intersystem crosses to the triplet state in less than 200 fs due to a sizable energy barrier of ca. 5 kcal/mol that must be surmounted to access the intramolecular charge-transfer state. Our results lend support to the idea that the probability of population transfer to the triplet manifold in these nitronaphthalene derivatives is controlled not only by the small energy gap between the Franck-Condon singlet excited state and the receiver triplet state but also by the region of configuration space sampled in the singlet excited-state potential energy surface at the time of excitation. It is proposed that the ultrafast intersystem crossing dynamics in these nitronaphthalene molecules most likely occurs between nonequilibrated excited states in the strongly nonadiabatic regime.

Nitronaphthalene derivatives (NNDs) are among the most abundant volatile nitro-polycyclic aromatic hydrocarbons found in the Earth's atmosphere. Investigations of the atmospheric degradation processes show that photolysis is the major degradation pathway under ambient conditions. In this contribution, we present photochemical measurements and quantum-chemical calculations of three major NNDs. It is shown that the magnitude of the photodegradation and triplet quantum yields in 1-nitronaphthalene (1NN), 2-methyl-1-nitronaphthalene (2M1NN), and 2-nitronaphthalene (2NN) are inversely related to each other. In accord with a recent time-resolved and computation study (J. Phys. Chem. A 2013, 117, 6580) and in order to explain this striking observation we propose that these photochemical yields are largely controlled by (1) the conformational heterogeneity of the nitro-aromatic torsion angle, (2) the energy gap (spin-orbit coupling interaction) between the excited singlet state and the receiver triplet state, and (3) the topology of the excited singlet state in the Franck-Condon region of configuration space sampled at the time of excitation. A distribution of torsion angles closer to 90° leads to a higher photoreactivity. Methylation of the ortho position in 1NN to give 2M1NN increases the photoreactivity by 97%, while 2NN is largely photoinert. Conversely, the triplet yield decreases as the distribution of torsion angles gets closer to 90°: 0.93 ± 0.15 in 2NN, 0.64 ± 0.12 in 1NN, and 0.33 ± 0.05 in 2M1NN. These results suggest an important relationship between conformational heterogeneity and the photochemical fate of these NNDs. This structure-photoreactivity relationship is of relevance to current efforts aimed at modeling and understanding the distribution patterns of NNDs in the atmosphere and their overall contribution to air quality.